A view of Jezero crater, the landing spot for the Perseverance roverNASA / JPL-Caltech / University of Arizona

The very first time NASA put a robot on Mars, it was looking for one thing: life.

When scientists examined the mission’s early findings, they sent for champagne. They were convinced they’d discovered proof, quietly metabolizing, in the soil. But the data turned out to be muddled, and one experiment designed to detect organic molecules found none, not even simple ones that astronauts had found on the moon.

Nearly half a century after that mission, its latest successor, a rover called Perseverance, left Earth this morning to scour Mars’s surface for signs of alien microbes.

The thought of the red planet bustling with alien beings has fascinated scientists for centuries. Some imagined a landscape shaped by an intelligent civilization—until telescopes became sophisticated enough to detail the planet’s surface. Others thought they saw dark swaths of vegetation, even a seasonal bloom—until orbiters revealed that the shadows were just shifting dust. By the mid-1970s, when the Viking mission launched, the only aliens Earthlings thought of encountering were microbes, which scientists had found could survive in Mars-like conditions simulated in labs. This new rover is the biggest and most technologically sophisticated spacecraft NASA has ever sent to Mars to answer one of the oldest questions: Does life exist anywhere but Earth?

Should the Perseverance rover find it, this time there will be no premature celebration: The evidence will come back to Earth for examination first. And so this search for life comes with a twist. The mission is designed to look for signs of only ancient microbes—extinct, fossilized, hollowed out—not present life, squirming away at this very moment.

Finding live aliens, even microbial ones, would force NASA to confront questions about interference and contamination; ancient life has the advantage of being both more likely to exist and inert. “Our best chance of identifying life on Mars would be going into the past,” says Lori Glaze, the director of NASA’s planetary-science division. Such a discovery would certainly be bittersweet, even depressing. Our corner of the solar system would seem less lonely, but the only neighbors we spotted would have turned out the lights long before we started knocking on their door.  

Several billion years ago, Mars was likely warm and sloshing with water. Orbiting spacecraft have found the imprints of dried-up waterways; the Curiosity rover, America’s previous emissary, dropped into a crater and found evidence of an ancient lake, with traces of organic molecules hidden in its soil. Perhaps microbes proliferated in that muggy environment, as they did on Earth.

Today, Mars is not a pleasant place, at least not for life as we know it. It is cold and dry. With a wispy atmosphere and no magnetic field, its surface is zapped by radiation.

The Perseverance rover will land inside a crater called Jezero, where, between 3 billion and 4 billion years ago, a river flowed into a lake the size of Lake Tahoe. On Earth, the geological record of the planet’s earliest days has been crumpled under shifting tectonic plates and sandpapered by weather. On Mars, including in Jezero, it is well preserved, pristine even, ready for a rover to come by and poke at it, searching with its robotic eyes for traces of dead microbes in the mud.

Perseverance will drill into rocks that seem promising, seal the soil it collects in little tubes, and deposit the samples on Mars’s surface. In the coming years, various machines are expected to descend on the planet to facilitate the cosmic relay race that comes next. In 2028, another rover will fetch the tubes and transfer them to a nearby rocket, which will launch to an orbiter circling overhead, becoming the first rocket to lift off another planet. The orbiter will grab the samples and head back to Earth, where it will drop them through the atmosphere and eventually into the labs of eager scientists. If this ambitious sequence goes as planned, NASA, together with the European Space Agency, will have transported martian dirt to Earth for the first time.

Scientists are especially hopeful that Perseverance will come across stromatolites, rocky mounds shaped like fairy-tale castles that, on Earth, are sometimes the work of microbial organisms. “Those are the holy grail,” says Briony Horgan, a planetary-science professor at Purdue University and a Perseverance scientist who led the selection of Jezero crater as the mission’s destination.

The tricky thing about this kind of investigation, though, is that some of the interesting phenomena scientists have detected on Mars that could indicate the presence of hidden life-forms—spikes in measurements of methane gas, mysterious shifts in the concentration of oxygen in the atmosphere—could also be explained by entirely abiotic, boring mechanisms. The same goes for Perseverance’s holy grail. “Not everything that looks like a stromatolite on Earth does in fact contain microbes,” Horgan says. And scientists back on Earth won’t know without getting a closer look in their own labs. “It’s going to be challenging to come up with a definitive answer with just the remote observations,” Glaze says.

Mars scientists—the most optimistic ones, at least—believe that if life exists anywhere near the surface right now, it dwells in the icy water deposits near the planet's poles. One explanation for why NASA hasn’t yet found any evidence of extant life on Mars is because, well, the agency hasn’t really been looking for it since the Viking mission in the ’70s.

In 2008, the Phoenix spacecraft landed in one of these spots, in the northern plains, to study the nature of water on Mars. Scientists actually spotted what appeared to be some droplets on its exterior—tiny translucent half-moons resting on metal—but the mission wasn’t intended to detect extant life.

Neither is the Perseverance rover. In fact, the rover isn’t supposed to go near places where extant life-forms have any chance of existing.

“The landing site for Perseverance was very carefully chosen to avoid areas where life might be living today,” Horgan says. “If we do find liquid water, that will pretty fundamentally change the mission.”

Perseverance will roam near Mars’s equator, where water ice resides deep underground, away from the warmth of the surface. The rover’s drills aren’t designed to reach it. The mission’s no-trespassing edict comes from NASA’s office of planetary protection, the organization charged with protecting the rest of the solar system from earthly contamination and vice versa.

Robotic missions that venture into regions on Mars where extant life could reside must abide by stricter cleanliness protocols, which Horgan says are onerous enough to make launching missions next to impossible. Even though Perseverance is headed toward a lifeless equatorial spot, engineers still had to sterilize every nut and bolt before launch. The dangers that even tiny packages of genetic material can pose are that much more apparent in this moment: The rover has a plaque affixed to it that honors health-care workers fighting COVID-19. If Perseverance somehow happened upon living martian microbes, scientists would have to face a host of ethical questions about what to do with them.

Still, the discovery of a tiny dead microbe would offer a granule of hope. “If we find evidence of ancient life, there’s a good probability that we might find extant life on the planet somewhere as well,” Sarah Stewart Johnson, a planetary scientist at Georgetown University, told me. “Life is so resilient, at least life as we know it. It can really wedge into a crevasse and hold on against all odds.”

Whatever they find in those samples, the next step in this long search could be to look, once again, for living beings. Carol Stoker, a planetary scientist at NASA, wants to return to Phoenix’s old landing site, and she has proposed a mission called Icebreaker to do exactly what it sounds like. Just a few million years ago, Mars’s orbit was tilted enough to expose the region to more sunlight than usual, enough to melt icy reservoirs into salty, liquid water where microorganisms could coalesce. The area would be frozen now, but the life within it might not have died off yet, Stoker says.

“We have many, many locations on Earth where you can find bacteria that are frozen in ground ice or in salt crystals that haven't been metabolically active for a million or a few million years, but can be revived,” Stoker says. “Put them into liquid water or an environment where they can wake up, and they do. They wake up and grow.”

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